The development of fuel cells is now under way actively from the viewpoint of environmental problems. There are known polymer electrolyte fuel cells, phosphoric acid fuel cells, molten carbonate fuel cells and solid oxide fuel cells according to the type of the electrolyte in use. Since polymer electrolyte fuel cells (PEFC) out of these are characterized by a relatively low reaction temperature and the easy fixing of an electrolyte, the use of PEFC as a small-sized home power supply, portable power supply or power supply for mobile objects is being started, and the development of PEFC for use in automobiles is now under way.
PEFC is a fuel cell which makes use of a phenomenon that a polymer resin membrane having a proton (hydrogen ion) exchange group in the molecule serves as a proton conductive electrolyte when it is impregnated with water until it is saturated. PEFC comprises a polymer electrolyte membrane which is composed of a polymer ion exchange membrane (cation exchange membrane) and a membrane-electrode assembly (fuel cell) which has an anode-side electrode and a cathode-side electrode arranged on the respective sides of this electrolyte and is sandwiched by a separator. A fuel gas supplied to the anode-side electrode, for example, hydrogen is changed into a hydrogen ion on a catalytic electrode to be moved toward the cathode-side electrode through the polymer electrolyte membrane which has been suitably humidified. An electron generated during this is taken out into an external circuit to be used as DC electric energy. Since an oxidant gas, for example, an oxygen gas or air is supplied to the cathode-side electrode, the above hydrogen ion, the above electron and oxygen are reacted with one another on the cathode-side electrode to produce water.
A perfluorosulfonic acid resin film (for example, Nafion (trade mark of E.I. DuPont De Nemours And Co.)) is used as the polymer electrolyte membrane and generally operated at a temperature of about 50 to 100° C. to reduce the resistivity of the polymer electrolyte membrane so as to obtain high power generation efficiency. The improvement of conductivity and the reduction of costs are required for this polymer electrolyte membrane, and it is hard to handle this polymer electrolyte membrane because it is a very thin film-like material. Therefore, when the polymer electrolyte membrane is joined to the electrodes or when a plurality of electric cells are stacked one upon another to obtain a laminate, the peripheral portion of the polymer electrolyte membrane wrinkles very often. Even when there are no wrinkles at all, the electrolyte membrane has the lowest mechanical strength among the constituent members of the stack.
To cope with this, JP-A 7-65847 proposes a reinforcing frame for reinforcing an electrolyte membrane mechanically and preventing a leak of a fuel gas or an oxidant gas from the interface with the electrolyte membrane. A reinforcing frame having desired mechanical strength and corrosion resistance even at the operation temperature is preferred, and polycarbonates, polyethylene terephthalate, glass fiber reinforced epoxy resin, titanium and carbon are disclosed as examples of the material of the reinforcing frame. Although a polycarbonate reinforcing frame is used in Examples of the above publication, when a polycarbonate is used, it is excellent in heat-resistant dimensional stability at an operation temperature of about 50 to 100° C. but inferior in mechanical strength.
JP-A 10-199551 proposes the use of a frame membrane having airtightness in the peripheral portion of a porous material fixed on the both sides of an electrolyte membrane. Polycarbonates, ethylene propylene copolymer, polyesters, modified polyphenylene oxide, polyphenylene sulfide and acrylonitrile styrene are enumerated as the material of the frame membrane.
When the materials proposed in these documents are made thin, their reinforcing effect degrades. They also have a defect that their reinforcing effect is not satisfactory at the operation temperature of PEFC.
JP-A 2007-103170 and JP-A 2007-250249 propose the use of polyethylene naphthalene dicarboxylate as a reinforcing member for the electrolyte membrane of PEFC. However, these documents were laid open after the priority date of the present application.
(Patent Document 1) JP-A 7-65847
(Patent Document 2) JP-A 10-199551
(Patent Document 3) JP-A 2007-103170
(Patent Document 4) JP-A 2007-250249